Tuesday, July 24, 2012

Once upon a time, NACA engineer Charles Zimmerman postulated that a very low-aspect-ratio wing would provide low-speed lift and short takeoff and landing performance. The wing ultimately became a disc with large propellers mounted at the forward outboard edges and rotating in opposite directions to the wingtip vortices. These made the wing even more effective at low speeds and increased the effective aspect ratio for cruise. Wind tunnel testing established the feasibility of the concept. Zimmerman joined Vought in 1937 where he designed and flew a model that demonstrated the configuration's potential for very low speed flight. Vought proposed a concept demonstrator, its V-173, to the Navy, which was intrigued enough to provide Vought with a contract for one in May 1940.

The V-173 was not small but fortunately it was light, a wooden and metal frame covered by fabric, since it was only powered by two 80-hp Continental engines.

Its first flight was on 23 November 1942. It was almost its last, since Boone Guyton found it difficult to get it turned back around to return for landing. Control modifications were made before its next flight that provided better handling qualities.

The Navy had already ordered a production version, the F5U. It was exactly the same size as the V-173, but of all-metal construction and powered by two 1,350-hp P&W R-2000 engines.

The mockup had three-bladed propellers but it was soon apparent to the engineers that the installed power and necessary rotor-hub gimbal (the blades had to "flap" to relieve root bending loads) dictated a design with four blades.

The development of the propellers and qualification of the gearboxes that interconnected the engines and the propellers delayed the program. It may have made its first low-power ground runs with F4U propellers. (The left-hand one appears to have the propeller blades rotated 180 degrees in the hub.)

The F5U's propellers had a much larger root chord and the opposed blades were paired. Note that the blade pairs were also staggered fore and aft.

The F5U was finally ready to fly in March 1947 but by then it was apparent that new fighters would be jet propelled. Although the F5U reportedly made short hops down the runway at Sikorsky Field, a first flight was not accomplished before the Navy terminated the program and directed that the prototype be destroyed.

Fortunately for aviation enthusiasts, Vought donated the V-173 to the Smithsonian. It languished at the Silver Hill storage facility for many years before being shipped to Grand Prairie, Texas where Vought Heritage volunteers restored it to like-new condition. It is now on display at the Frontiers of Flight Museum in Dallas, Texas. See http://www.flightmuseum.com/

When an aircraft carrier deploys, it departs with an air
wing comprised of several specialized squadrons to provide a full offensive and
defensive capability. The exact
composition of the air wing has varied over the years, but in addition to
fighter and attack squadrons, there are always airborne early warning and
reconnaissance airplanes as well plane guard/utility helicopters. Now
electronic warfare airplanes are a standard part of the air wing.

Fighter and attack squadrons were typically assigned to a
specific air wing for an extended period of time. At one point, in the late
1950s and early 1960s there were usually three fighter squadrons per air wing even
though only two were required for a deployment. Because of the rapid
development of new or improved fighter types at the time, one of the three was
likely to be in transition to a new type and not ready to deploy.

For various reasons, a squadron might not be ready for
deployment with its air wing and a substitute was required. Often, this would
be a Marine Corps squadron since its pilots would all be naval aviators,
qualified for carrier takeoffs and landings. VMFA-333 deployed several times, for example flying F-4s with Carrier Air Wing Eight aboard Nimitz in 1976.

One very non-traditional Marine Corps squadron deployment took place on the last cruise of Roosevelt. VMA-231, flying AV-8A Harriers, was assigned
to Carrier Air Wing 19 for its cruise in the Mediterranean from October 1976 to
April 1977. Also aboard were two squadrons of F-4Ns and three of A-7Bs, along
with detachments of E-1Bs (for the last deployment of this type), RF-8Gs, and SH-3Gs.

In the mid 1970s, the Navy was seriously evaluating a transition to
V/STOL aircraft for all sea-based, manned, tactical air missions instead of building more big
aircraft carriers equipped with catapults and arresting gear. In early 1976, the CNO briefed OSD on a tentative plan to do so. The assignment of VMA-231 to the
Roosevelt’s air wing was intended to provide insight into the feasibility and
benefits of a operating a V/STOL fighter/bomber at sea.

The Harrier had been in service with the Marine Corps since
1971 and had already been evaluated in an extended series of at-sea trials
aboard, among others, the amphibious assault ship Guam (LPH-9) that was serving as an Interim Sea Control Ship. This
resulted in the development of a corrosion control plan for an extended
deployment among other operational procedures. However, there were still
concerns about the Harrier’s maintenance requirements, hot exhaust, lack of
endurance, etc.

VMA-231 worked up to the deployment via a series of
mini-cruises aboard Franklin D. Roosevelt
beginning in late June 1976. These established operating procedures and
familiarized the ship’s company with the unique characteristics of the Harrier,
like the downward-directed engine exhaust in VTOL mode.

U.S. Naval Institute Proceedings October 1977

V/STOL advocates considered the experiment a virtually
unqualified success. Complying with standard carrier cyclic operations (90-minute flight period for the conventional takeoff and landing airplanes) proved to be unnecessary since the Harriers could land in any open space during a launch/land cycle. Benefits demonstrated early on included no time or crew required to hook up to the catapult for takeoff, virtually no waveoffs (and zero bolters), and the ability to back into a designated parking space. The Harriers could also land with the ship steaming out of the wind in conditions that precluded the operation of its conventional airplanes.

Rolling takeoffs were a bit more problematical in some wind over deck conditions but a vertical takeoff was almost always possible. Flight time, however, was limited to 20 minutes by the reduction in fuel required.

Over 2,000 sorties and landings, 15% at night, were accomplished by VMA-231 during the deployment. There were no aircrew or aircraft losses, a non-trivial accomplishment given the accident rate of carrier operations. The promise of V/STOL seemed to have been clearly demonstrated and V/STOL aircraft welcome aboard.

In parallel with and supported by this evaluation, the Navy initiated a set of V/STOL development programs, with V/STOL A being a subsonic multi-mission aircraft and V/STOL B, a supersonic fighter and attack aircraft. (There was also a V/STOL C, which was to be a smaller ASW aircraft to replace the LAMPS Mk III helicopter operating from frigates and destroyers.)

Except for Bell's tiltrotor and Sikorsky's ABC, the V/STOL A concepts used various forms of lift fans and/or high-bypass-ratio jet engines like this Vought proposal.

The V/STOL B designs were equally innovative, in some cases using lift fans for more efficient vertical lift like this McDonnell Douglas concept.

After only a couple of years of studies, however, the Navy reversed course after completing its Sea-Based Air Master Study in 1980. It
concluded that an all-V/STOL approach incorporating existing technology was high risk and in any event would
cost more than utilizing conventional airplanes.

Two aircraft, the V-22 Osprey and the F-35B, did eventually result from the Navy's flirtation with V/STOL, however. The tiltrotor had been proposed for V/STOL A and thereby attracted the attention of the Marine Corps. The F-35B resulted from follow-on research programs to advance V/STOL technology. It's therefore possible that the U.S. Navy might yet transition to V/STOL carriers...

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In 1956, at age 12, I lived on NAS Sangley Point in the Philippine Islands. Always enamored with airplanes, I imprinted on the Cougars, Banshees, and Skyraiders then being deployed. Not able to be a Naval Aviator because I was nearsighted, I instead became an aeronautical engineer and general aviation pilot. Now retired, I write books and monographs on U.S. Navy aircraft.